Love is a matter lacking any importance (7)
I believe the answer is:
nothing
'love' is the definition.
(love means zero in tennis)
'matter lacking any importance' is the wordplay.
I cannot really see how this works, but
'matter' could be 'thing' (synonyms) and 'thing' is found within the answer.
'lacking' could be 'no' (eg lacking money means with no money) and 'no' is found within the answer.
No letters remain.
This explanation may well be incorrect...
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for nothing that I've seen before include "Zilch, nada" , "signifying love" , "Naught, nil, nix" , "What's in a vacuum?" , "'Zero, zilch (7)'" .)